1. Field of the Invention
This invention relates generally to the field of subsea oil and gas operations and more specifically to a bulls eye indicator and method, which may be used to measure the angle of inclination of subsea structures on the ocean floor.
2. Background of the Invention
A bulls eye indicator is a type of inclinometer. An inclinometer may also be called, in some circumstances, a clinometer, tilt meter, tilt indicator, slope alert, slope gauge, gradient meter, gradiometer, level gauge, level meter, declinometer, and pitch & roll indicator. Clinometers measure both inclines (positive slopes, as seen by an observer looking upwards) and declines (negative slopes, as seen by an observer looking downward). Astrolabes are inclinometers that were used for navigation and locating astronomical objects. Inclinometers have been used in various applications including airplane instruments, automobile instruments, and marine operations. A floating ball inclinometer has been used in personal vehicles to gauge the angle of inclination of the vehicle and prevent vehicle rollovers and injuries. In one type, a ball with degree markings is supported in a fluid filled transparent container. The instrument provides a reading based on the center of gravity of the vehicle.
The bulls eye indicator, or marine inclinometer, is a device for measuring slope on the sea floor and is commonly used in underwater oil and gas operations. The bulls eye indicator gets the name because the grid lines are generally concentric circles and appear as a bulls eye type target. The apparatus is critical in determining how to properly position underwater equipment and components on the uneven sea floor. This art is commonly used to level subsea blow out preventor stacks, suction piles, subsea Christmas trees, and the like.
The device is typically a cylinder shaped container and a base marked with a bulls eye inclination indicia. The degree markings may be stenciled on the base for viewing through the window. A metal ball inside the container moves based on the inclination and is measured against the degree markings to determine the slope of the underwater device upon which the device rests. The bulls eye indicator instrument is typically secured to the equipment that must be leveled, typically prior to lowering the equipment to the ocean floor.
The device is pressure tested to withstand the sometimes very high water pressure at the depths of the ocean. The bulls eye indicator results are typically viewed by underwater cameras on remotely operated vehicles (ROV) and then transmitted to the surface so that real time leveling adjustments may be made as the equipment is leveled. The bulls eye detector comes in a variety of sizes and angles of inclination which can be measured. The bulls eye detector may provide multiple methods to secure the instrument, including a fixed mounting plate, a leveling base, and may be ROV retrievable.
The inventor has found that while the prior art bulls eye instrument is potentially very accurate mechanically, a common problem with subsea inclinometers is that of parallax error. Parallax error is a distortion caused by the observer and the object being observed being in different lines of sight, due to light waves bending when encountering two or more different mediums, causing inaccurate readings. Accordingly, a typical problem involves the ball used in the instrument being perceived by the observer at a different angle of inclination than the ball truly indicates.
Another problem is that of debris or silt buildup on the window, which may be problematic if the debris cannot be removed. Due to debris, the longer the instrument remains underwater, the more difficult the debris problem becomes to interpret the readings. Prior art inclinometers lack any defense to silt buildup.
The following patents show background prior art related to inclinometers generally.
U.S. Pat. No. 4,349,809, to M. B. Tomes, issued Sep. 14, 1982 discloses an apparatus for detecting the angle of inclination of a vehicle and for providing an alarm signal when the inclination of the vehicle exceeds a predetermined maximum safe level. An elongate, pliant tubular member made of an electrically conductive material has its intermediate segment mounted by an insulative clamp to a vertical panel which may be situated on the vehicle's dashboard. The end segments of the tubular member are upwardly inclined, and the degree of inclination is shown by angular indicia on adjacent portions of the vertical panel. A plurality of spherical contact elements made of an electrically conductive material are positioned within the tubular member. A pair of contact points made of an electrically conductive material are inserted into the opposite ends of the tubular member. When the angle of inclination of the vehicle exceeds the safe level, the contact elements will strike one of the contact points. This causes an electric current from the vehicle's battery to energize an indicator such as a buzzer which provides a warning signal.
U.S. Pat. No. 4,435,108, to J. E. Hampton, issued Mar. 6, 1984, discloses a method of installing sub-sea templates. A subsea template is installed by a method which includes the steps of securing the template in a position beneath the deck of a semi-submersible drilling vessel, moving the semi-submersible drilling vessel to an appropriate offshore site and subsequently lowering the template from the semi-submersible to the sea bed. In addition, at least three anchorage templates may be loaded onto one or both of the pontoons of the semi-submersible drilling vessel at its original position and are subsequently lowered from the pontoons to their respective locations on the sea bed after the semi-submersible has moved to the offshore site.
U.S. Pat. No. 6,631,916, to M. E. Miller, issued Oct. 14, 2003, discloses a guidance system for pull type equipment. An improved equipment guidance system and assembly is described comprising a top plate in releasable engagement with a drawbar of a towing vehicle, a supplemental or ancillary drawbar pivotable about a forward location and movable to an offset position left or right of center by a pair of pistons, and a bottom plate for attachment to the ancillary drawbar and the top plate. The pistons are controlled by a control box that dictates in concert the extension and retraction of the left and right pistons. A signal from the control box is generated from a plurality of sources. One example of a signal source is a tilt sensor that measures inclination relative to gravity. Another example of a signal source is an automatic “whiskers” crop row sensing wand system. Yet another example of a signal source can be generated manually from an operator of the towing vehicle.
U.S. Patent Application Publication 2005/0017139, to C. A. Tacklind, published Jan. 27, 2005, discloses an apparatus including a surface secured to at least one leg, and a leveling apparatus secured to the platform providing an indication of level about at least two axes. A method of positioning the apparatus includes the steps of positioning an end point of a first leg on a fiduciary point; setting a height of a mounting platform by extending said first leg; positioning an end point of a second leg to engage a surface; leveling the mounting platform on a first axis; positioning an end point of a third leg to engage said surface; leveling the mounting platform on a second axis; and repositioning said first leg so that said end point is in desired location.
The above discussed prior art does not solve the aforementioned long standing problems. Accordingly, those of skill in the art will appreciate the present invention which addresses the above and other problems.